In recent years, DC motors associated with a field winding have been driven by using a three-phase anti-parallel thyristor bridge circuit, which has an advantage that the return of energy from the DC motor to the main circuit and the reverse rotation of the DC motor can be effected without change-over contacts. The three-phase anti-parallel bridge circuit comprises a first bridge circuit formed by six thyristors for supplying forward current to the DC motor and a second bridge circuit formed by six thyristors for supplying backward current to the DC motor. The first and second bridge circuits are connected in parallel to each other with the conducting directions of the thyristors between the two circuits opposite each other. The conducting state of the first bridge circuit is called a "converter operating state", while the conducting state of the second bridge circuit is called an "inverter operating state". When the speed of the DC motor is decreased, the three-phase anti-parallel bridge circuit is changed from the converter operating state into the inverter operating state, and, after that, the anti-parallel bridge circuit is again changed into the converter operating state. During those changes of the state of the anti-parallel bridge circuit, the commutation between the thyristors of the first bridge circuit and those of the second bridge circuit may fail, if pulses for firing the thyristors are not generated or if a three-phase alternating voltage applied to the thyristors is decreased. As a result, an excess of current flows through the DC motor so that the DC motor may be damaged. Therefore, it is important to avoid the failure of the commutation of the thyristors between two bridge circuits connected anti-parallel to each other. The present invention relates to avoiding the failure of commutation in the case of an applied voltage drop.